Humble Bundle BTA Prices

if a lot of people buy 1$ bundle, bta price go up i think

Differs, seen it go up and down. This one, so far only has been raising.

The big Humble Bundles (not the weekly ones) have a flexible BTA price. That means that the BTA price starts with a custom value and goes up or down, depending on the average of the prices paid by the subsequent buyers. If a lot of people buy the 1$ value, the BTA will go down, as the average price paid will go down. If prices paid for the bundle are usually high, BTA goes up.

From my own experience, Bundles start cheapest, and get marginally more expensive (maybe around $1 more) in the following hours. A trend of prices is then established, and each buy will have less effect on the BTA, so it won't change that much. Bottom line, get the bundle in around half an hour after it's been on for the best BTA deal.